During processing of silver halide photographic elements, the developer is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions which are more compatible with the environment, and thus it is desirable to reduce or avoid the use of ferric ions and many of the common complexing ligands such as ethylenediaminetetraacetic acid and propylenediaminetetraacetic acid, which are not readily biodegradable.
Peroxide bleaching solutions, such as those containing hydrogen peroxide, or a peroxide precursor such as perborate or percarbonate, bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive, and present lower chemical and biological demands on the environment since their by-products can be less harmful.
However, there are certain problems associated with peroxide bleaching solutions. For example, acidic peroxide bleaching solutions are described in U.S. Pat. No. 4,277,556 (Koboshi et al) which may contain metal ions as catalysts to improve the bleaching efficiency of the peroxide. Some of these solutions are stable, but they are not rehalogenating or silver retentive (that is, the developed silver can be oxidized to a soluble salt, and undeveloped silver halide remains in the element). This complicates silver recovery. Other peroxide bleaching solutions are known which contain a ferric ion complex as a catalyst, as described in U.S. Pat. No. 4,301,236 (Idota et al). Such solutions are rehalogenating, but lack stability as iron is a well-known catalyst for peroxide decomposition. In addition, the only examples show the use of EDTA which is undesirable because of its non-biodegradability.
Thiols have been described as bleaching catalysts in some literature (JP-A-61/261739), but no demonstration of their effect was provided. Nonetheless, thiols are undesirable as catalysts because of their objectionable odors.
EPA-0 428 101 describes alkaline peroxide bleaching solutions which are rehalogenating, but such solutions lack sufficient stability and are claimed to bleach photographic elements containing silver chloride only.
Other concerns with peroxide bleaching solutions include the potential for vesiculation, that is the blistering in the element caused by the breakdown of peroxide into water and oxygen gas.
There remains a need, therefore, for highly efficient peroxide bleaching solutions which do not suffer from the problems noted above, that is, they are rehalogenating, stable and useful for a variety of photographic elements, and lack objectionable odors.